Aram Dorsum was the other candidate on the table during the two days of discussions at Esa’s technical centre in Noordwijk in the Netherlands. Aram contains a large channel. The sedimentary rocks around this channel may well be the consequence of flood deposits.

But in the end, the Noordwijk meeting voted in favour of Marwth, and this was then endorsed by the ExoMars Landing Site Selection Working Group (LSSWG).

Technically, Oxia Planum – another clay-rich location – was back in competition, but it remains in play and will now be subjected “excruciating investigation”, as ExoMars project scientist Jorge Vago put it, along with Mawrth.

“In Oxia and Mawrth, we will be investigating areas of Mars where the deposits are so ancient they record what the conditions were during the very early history of the planet – more than four billion years ago,” Dr Vago told BBC News.

“No other mission has landed on site this old. So, I think the mineral variety and the age of the landing site contributed to the preference of the people who voted for Mawrth Vallis.”

Picture analysts need to identify a swathe of ground roughly 100km by 15km (the ellipse of error expected with the rover’s landing system) at both locations that is largely free from fissures, excessive slopes and large boulders.

These are the hazards that could kill the rover before its surface mission has even begun.

Assuming that can be done for both Oxia Planum and Mawrth Vallis, a final decision on where to send the robot can be made on the science imperatives alone.

This downselection to the one preferred destination is not expected to be made until the year before launch.

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